This invention relates to frequency modulated oscillators. Present art direct frequency modulated crystal oscillators using variable capacitance diodes can be divided in two main categories or types. The term, "direct frequency modulated crystal oscillator" refers to that type of modulation frequency with or without temperature compensation where the direct frequency modulation is obtained by applying a modulating signal, such as an audio signal, to a voltage variable capacitance diode (varicap) biased by a fixed D.C. voltage network.
In a first type of modulated crystal oscillator the required system frequency deviation is obtained by further multiplication of oscillator frequency deviation, usually by a factor of nine to 36 times. In this type of system, the total frequency excursion due to FM modulation is a fraction of the spacing between the zero and the pole frequencies of the crystal.
In the second type of system, the oscillator is also modulated in frequency by applying the audio signal to a D.C. biased voltage variable capacitance diode, but the total system frequency deviation is obtained directly without further multiplication from the crystal oscillator. In land mobile radio applications, the system peak frequency deviation is limited to 5 KHz above and below the transmitter carrier frequency that span, at present in the U.S., the frequency range of 25 to 512 MHz. Because a total of 10 kHz excursion of the crystal frequency is too large a portion of the crystal pole-zero frequencies spacing, the oscillator frequency deviation vs. modulating signal amplitude characteristic becomes non-linear with consequent increase of distortion of the recovered audio signal. The linearity is improved and the frequency deviation for a given modulating signal amplitude is increased if an inductor is inserted in series with the crystal and the feedback capacitances of the circuit.
Analysis of such circuit shows that the inductor improves the linearity of the oscillator characteristic of frequency deviation versus modulation voltage amplitude, permits small adjustment of the crystal output frequency to offset manufacturing crystal calibration and aging tolerance, and increases the frequency deviation sensitivity (the oscillator frequency output excursion for a given modulating voltage amplitude).
These three functions are interdependent; therefore, the adjustment of the inductance value that improves the linearity and the distortion most of the time does not always coincide with the required value necessary to set the frequency or to obtain the desired constant sensitivity necessary. This is especially true in multifrequency applications when driven by the same audio signal source. The usual solution to these problems is invariably a compromise; i.e., the inductor is adjusted for the value that permits the setting of the frequency by compensating for the overall feedback capacitance and manufacturing crystal tolerances, and in the process offers a value that somewhat improves the linearity. The characteristic that the compromise sacrifices is the ability to set the frequency deviation sensitivity precisely in multifrequencies applications. In the latter case, the equalization of the frequency deviation in a given frequency range is obtained by applying the audio signal through an adjustable potentiometer that can set a voltage different in amplitude for each oscillator in order to compensate or equalize different oscillator frequency deviation sensitivities.
The disadvantages of the prior art type of circuit are the following:
1. It requires a potentiometer for each frequency channel element to equalize the different frequency deviation sensitivities of each individual channel oscillator.
2. It requires an inductor of low Q in series with the frequency determining elements of the crystal network and therefore the noise and frequency stability are now dependent also on the stability of the inductor characteristics.
Usually both noise and frequency stability are degraded with respect to those of the crystal and feedback stable capacitors alone. The degradation is proportional to the amount the crystal series resonance frequency is shifted down by the inductor value. Since this value is unique for a given frequency and linearity improvement, no minimization of this effect is possible without affecting the demodulated signal distortion.
3. A certain degree of instability and/or frequency jumping exists, created by the inductor-generated new reactance zero above the crystal pole frequency where usually large crystal spurious frequencies can produce unwanted oscillation, either crystal or non-crystal controlled.
4. The slope of the frequency deviation versus the modulating voltage V.sub.D is not constant, as it ideally should be, but has a constant negative slope which gives different frequency deviations for equal negative or positive going modulating signal, a phenomenon called frequency deviation asymmetry. This lack of symmetry between positive and negative going frequency excursions when driven by symmetrical modulating input is the most serious drawback of the prior art approaches. Another limitation is that when a frequency-temperature compensation voltage is to be applied directly at the modulating element, it affects the symmetry and the sensitivity of the frequency deviation. The latter is a very important characteristic when the oscillator should be compensated to frequency stability of better than .+-. 2 ppm in the temperature range of -40.degree. to +80.degree. C.